Nanotechnology and microrobotics are emerging as innovative tools in environmental remediation, particularly in water treatment, where pollutants are continuously increasing and are increasingly less sequesterable with traditional methods.

One of the greatest global health risks stems from access to impure water, whether it’s the water discharged into rivers and seas after treatment in specialised plants or the water that reaches our homes. Waterborne diseases caused by bacteria in contaminated water have a devastating impact on public health. For instance, in the United States, Escherichia coli alone is responsible for approximately 265,000 infections annually. Furthermore, diseases such as hepatitis, typhoid, and cholera claim lives in socio-economically vulnerable areas [source: World Health Organization].

With bacteria, microplastics, heavy metals, and chemical pollutants present in wastewater – substances that are often challenging to remove completely due to their composition and resistance to biodegradation – the scarcity of clean water is becoming increasingly severe. Contributing factors include a continuously growing global population (which increased from 7,909,295,151 in 2021 to 8,118,835,999 in 2024), the effects of climate change, and the intensification of industrial processes in certain parts of the world.

Remarkably, a 2019 study by the German Helmholtz Centre for Polar and Marine Research and the WSL Institute for Snow and Avalanche Research in Switzerland found microplastics in Arctic and Swiss Alpine snow, with predominant materials including paint, rubber, polyethylene, and polyamide, likely transported by the atmosphere [“White and wonderful? Microplastics prevail in snow from the Alps to the Arctic” – Science Advances, August 2019].

Nanotechnology’s contribution to wastewater treatment extends beyond nanomaterials and nanoparticles capable of eliminating numerous contaminants from aquatic environments; it also includes the design of micromachines for the rapid sequestration of pollutants.
A recent study by the Brno University of Technology in the Czech Republic proposes a swarm of polymeric and magnetic microrobots, with a diameter of 2.8 micrometres, designed to remove both microplastics and bacteria from the same aquatic medium for the first time.
In the future, these microrobots could evolve into microstructures capable of capturing other types of pollutants, thereby addressing the need to purify wastewater from Emerging Pollutants (primarily antibiotics and hormones).

Microplastics and bacteria: the urgent pollutants to address

The complete removal of microplastics and bacteria from wastewater represents a significant environmental challenge of our time. Microplastics, which can be as small as 5 mm, result from the gradual fragmentation of all plastics discarded into the environment, including those recycled, landfilled, lost at sea during maritime transport, and inadequately treated in wastewater plants. According to the National Geographic Society, it could take hundreds or thousands of years for plastic to fully decompose globally.

The danger of microplastics lies in their ability to interact with other pollutants, becoming a transport medium and causing «widespread contamination within terrestrial and marine ecosystems, with significant repercussions for human, animal, and plant health» [“Effects of microplastics on the terrestrial environment: A critical review” – Environmental Research, 2022].

Bacteria, on the other hand, manifest in various states, including suspended microorganisms in liquids and biofilms. In the former, they multiply rapidly, degrading water quality and spreading bacterial infections; in the latter, «the formation of bacterial biofilms on industrial pipes and water lines slows water flow and corrodes pipes, thereby reducing the hydraulic efficiency of power plants and further compromising the safety of drinking water distribution systems» [source: “Smart micro- and nanorobots for water purification” – Nature Reviews Bioengineering, February 2023].

Regarding potable water, research detailed in “Safeguarding the microbial water quality from source to tap” (npj Clean Water, April 2021), conducted by the Center for Microbial Ecology and Technology at Ghent University, Belgium, and the European Centre of Excellence for Sustainable Water Technology, Netherlands, addresses its “biostability.” This stability is increasingly challenged by human activities and climate change. The research team emphasises that «until now, biostability has been maintained through the selection of high-quality water sources and timely use of disinfectants. However, as new contaminants continuously infiltrate freshwater, current strategies may no longer suffice to maintain a consistent and high-quality potable water supply in the future». It is time to start accounting for this reality.becoming less sequesterable by traditional methods.

Wastewater treatment technologies and emerging contaminants

So-called “wastewater” – essentially sewage water – typically originates from urban, domestic, agricultural, and industrial activities. It is collected in sewer networks and then directed to treatment plants, from where it is returned to the natural environment not as potable water but rather through direct discharge into rivers or seas, or through reuse applications in irrigation or industrial cooling systems.

Traditional treatment methods, designed to remove all known pollutants from wastewater, both soluble and insoluble, utilise physical processes such as filtration and membrane technologies, as well as chemical and biological processes.

Over the last decade, nanotechnologies have emerged in this sector, with the development of innovative nanomaterials for treating wastewater and also «contaminated groundwater, which contains metallic ions, organic and inorganic solvents, and microorganisms», as explained by two researchers from Banasthali University, India, authors of “Smart and innovative nanotechnology applications for water purification” (Hybrid Advances, August 2023).

Among the nanomaterials capable of eliminating numerous pollutants from water are those based on catalytic oxidation of moist air, zero-valent iron, photocatalysts, zeolite nanoparticles, and nanobubbles.

Concerning contaminated water, significant attention has been given to the work of a group of Indian researchers, published in December 2023 in the scientific journal Water-Energy Nexus (“Emerging pollutants of severe environmental concern in water and wastewater: A comprehensive review on current developments and future research“). This study discusses Emerging Pollutants (EPs), a new class of contaminants present in wastewater, mainly from the fertiliser industry, the pharmaceutical industry, and personal care products.

«The new contaminants», the team points out, «have significantly emerged due to the enormous global consumption of drugs, particularly antibiotics and hormones, and personal care products. They have both human and animal origins, with the ability to permeate waterways instantly and soil gradually, posing a risk of contaminating potable water sources».

These pollutants can have negative impacts on human health, «including the development of resistant bacteria, toxic neurological effects, and endocrine disruptions».

The crux of the matter is that existing wastewater treatment plants were not designed to purify these new contaminants. This is the key issue.

The role of microrobotics in wastewater treatment: some examples

The contribution of nanotechnologies to the field of wastewater treatment is also evident in the design of microsystems and micromachines aimed at the rapid and efficient capture of pollutants, where traditional systems often fall short.

One of the earliest examples is illustrated by an international study in 2016, conducted jointly by researchers from the Max-Planck Institute for Intelligent Systems in Stuttgart, the Institute for Bioengineering of Catalonia, and Nanyang Technological University in Singapore (“Graphene-Based Microbots for Toxic Heavy Metal Removal and Recovery from Water” – Nano Letters). They developed graphene oxide-based microrobots, used as active self-propelled systems for removing heavy metals like lead from water.

In 2017, the same team published another study (“Swimming microbots can remove pathogenic bacteria from water” – Science Daily), this time focused on eliminating bacteria from water without using disinfectants and without producing chemical waste. This was achieved through «microbots coated on one side with magnesium and on the other with alternating layers of iron and gold, topped with silver nanoparticles: bacteria adhere to the gold and are then killed by the nanoparticles. »

Another particularly stubborn and hard-to-decompose pollutant in wastewater is disposable tissues, typical of wet wipes («about 14,000 of which are used worldwide every second»), composed of modified cellulose and polypropylene. To break these down, a group of scientists from the University of Chemistry and Technology in Prague developed self-propelled microrobots made of bismuth and tungsten, supported by oxidative processes under light irradiation (source: “Swarming of Perovskite-Like Bi2WO6 Microrobots Destroy Textile Fibers under Visible Light” – Advanced Functional Materials, September 2020).

A year later, the same university published another research on the topic (“A Maze in Plastic Wastes: Autonomous Motile Photocatalytic Microrobots against Microplastics” – Applied Materials and Interfaces, May 2021), this time focusing on removing microplastics through photocatalytic degradation, based on «light-guided microrobots capable of capturing and degrading microplastics ‘on the fly’ in a complex multi-channel maze».

Swarms of microrobots to capture microplastics and bacteria in the same water sample

The latest development from Brno University of Technology in the Czech Republic, detailed by the authors in “Magnetic Microrobot Swarms with Polymeric Hands Catching Bacteria and Microplastics in Water” (ACS Nano, May 2024), introduces a microrobot model with a diameter of 2.8 micrometres (where one micrometre is one-thousandth of a millimetre). This model can, for the first time, simultaneously remove microplastics and bacteria from the same contaminated aqueous medium.

Specifically, these are swarms of magnetic microrobots controlled by orthogonal coils that generate rotating magnetic fields.

Disegno che descrive il piano rotante a propulsione magnetica con, al centro, i microrobot sferici (in giallo) dalle “mani” polimeriche sequestranti (prolungamenti azzurri) mentre agganciano microplastiche (le piccole sfere bianche) e batteri (in verde) [credit: “Magnetic Microrobot Swarms with Polymeric Hands Catching Bacteria and Microplastics in Water” - ACS Nano 2024 -].
[Credit: “Magnetic Microrobot Swarms with Polymeric Hands Catching Bacteria and Microplastics in Water” – ACS Nano 2024 –].

Within the swarms«the microbots communicate through magnetic interactions that, in turn, enable the coordinated movements of the micromachines in water, while still allowing each unit to maintain its structure and function», explain the researchers from Brno University.

1) Flusso di contaminanti (batteri di Pseudomonas aeruginosa e microplastiche) in un recipiente circondato da bobine ortogonali che generano un campo magnetico rotante; (2) auto-organizzazione di microrobot polimerici guidati magneticamente in piani rotanti e cattura dei contaminanti; (3) distacco dei batteri catturati; (4) raccolta magnetica di microrobot per il riutilizzo; (5) trasferimento dei contaminanti in un secondo recipiente per la disinfezione con irradiazione mediante luce UV [credit: “Magnetic Microrobot Swarms with Polymeric Hands Catching Bacteria and Microplastics in Water” - ACS Nano 2024 -].
The process includes: (1) flow of contaminants (Pseudomonas aeruginosa bacteria and microplastics) into a vessel surrounded by orthogonal coils generating a rotating magnetic field; (2) self-organisation of magnetically guided polymeric microrobots into rotating planes and contaminant capture; (3) detachment of captured bacteria; (4) magnetic collection of microrobots for reuse; (5) transfer of contaminants to a second vessel for UV light irradiation disinfection [credit: “Magnetic Microrobot Swarms with Polymeric Hands Catching Bacteria and Microplastics in Water” – ACS Nano 2024 –].

Each magnetic micromachine is coated with polymers, prepared using the monomer N-[3-(Dimethylamino)propyl]methacrylamide, which recent literature has deemed toxic to bacteria, demonstrating clear antibacterial efficacy in vitro. «This polymer has shown a significant ability to bind to bacteria through electrostatic interactions, performing exceptionally well against Gram-negative bacteria. In this context, it acted as a ‘sequestrant’ for bacteria, disrupting related chemical reactions», illustrate the researchers, who selected the bacterium Pseudomonas aeruginosa, known for its high resistance to disinfection, as the model for in vitro tests.

The Movement of the microrobot swarm

The central core of the microrobots forming the swarm is a sphere, described by the authors as “superparamagnetic“. When not energised externally by a magnetic field, the microstructures remain dispersed in the water.

«In the presence of an external rotating magnetic field, however, they align with the applied field, attracting each other along their magnetic dipoles. They assemble into planes, which then start moving under the influence of the rotating magnetic field and imposed directions. More precisely, they exhibit collective and coordinated movement, dynamically reacting to the influence of the magnetic field. This behaviour is known as a “swarm” in microrobotics and essentially describes «aerial microrobots» working together in synchronisation.

Capturing contaminants

Capturing free-swimming bacteria in water is not straightforward, especially considering their ability to form highly resistant biofilms on surfaces, which are difficult to dissolve. Additionally, the presence of microplastics complicates the removal of bacteria, as they serve as vectors for transporting various pollutants. 

To assess the practical efficacy of the designed polymeric magnetic microrobot swarms, the research team conducted an experiment to capture microplastics and Pseudomonas aeruginosa bacteria in the same water samples:

«The experimental setup involved introducing P. aeruginosa bacteria into an aqueous solution containing fluorescent polystyrene beads about 1 μm in diameter, used as a model for microplastics.The mixture was then subjected to a transverse rotating magnetic field of 5 mT and a frequency of 10 Hz for 30 minutes, during which the polymeric magnetic microrobots trapped the contaminants»

At the end of the test, the microbots and the sequestered contaminants were collected, and the water was analysed to estimate the residual bacterial concentration, which was found to be 0.3, slightly lower than observed in the absence of microplastic contaminants. Images from the experiment show the microrobots interacting with the contaminants, both microplastics and bacteria, as documented by the temporal dynamics of microplastic reduction, obtained through optical imaging using a fluorescence microscope.

Schema dell’esperimento, che illustra: (a) soluzione di batteri Pseudomonas aeruginosa mescolata con perline di polistirene fluorescenti (~ 1 μm di diametro) esposta ai microrobot sotto un campo magnetico rotante trasversale di 5 mT e una frequenza di 10 Hz per 30 minuti. Quindi, i microrobot con i contaminanti catturati vengono raccolti e rimossi dalla soluzione utilizzando un magnete permanente, (b) grafico che illustra i valori del residuo di Pseudomonas aeruginosa nella soluzione dopo la raccolta dei microrobot (utilizzati a una concentrazione di 7,5 mg mL–1) e i valori risultanti confrontati con la soluzione contaminata (blocco verde) e con la condizione di assenza di movimento (blocco viola), (c) immagini di microrobot raccolti magneticamente dopo aver sequestrato i contaminanti, (d) micrografie ottiche che mostrano la riduzione del modello microplastico in acqua a intervalli di 0, 15 e 30 minuti, (e) immagini time-lapse a diversi intervalli di tempo (0, 0,75, 1,5 e 4,5 s) di sciami microrobotici rotanti che trasportano le microplastiche catturate in acqua [credit: “Magnetic Microrobot Swarms with Polymeric Hands Catching Bacteria and Microplastics in Water” - ACS Nano 2024 -].
Experiment diagram: (a) solution of Pseudomonas aeruginosa bacteria mixed with fluorescent polystyrene beads (~1 μm in diameter) exposed to microrobots under a transverse rotating magnetic field of 5 mT and a frequency of 10 Hz for 30 minutes. The microrobots with captured contaminants are then collected and removed from the solution using a permanent magnet, (b) graph illustrating residual Pseudomonas aeruginosa values in the solution after microrobot collection (used at a concentration of 7.5 mg mL–1) compared to the contaminated solution (green block) and the no-motion condition (purple block), (c) images of magnetically collected microrobots after capturing contaminants, (d) optical micrographs showing the reduction of the microplastic model in water at intervals of 0, 15, and 30 minutes, (e) time-lapse images at various intervals (0, 0.75, 1.5, and 4.5 s) of rotating microrobot swarms transporting captured microplastics in water [credit: “Magnetic Microrobot Swarms with Polymeric Hands Catching Bacteria and Microplastics in Water” – ACS Nano 2024 –].

Glimpses of Futures

Demonstrating – albeit through an initial experiment (many more will be needed) – that it is possible to simultaneously eliminate bacteria and microplastics from the same aquatic environment, the study highlights the potential of microrobots in addressing complex environmental challenges, such as water purification in general, and wastewater treatment in particular.

Let us now try to anticipate possible future scenarios by analysing the impacts that the evolution of the described microrobots could have from a social, technological, economic, political, and sustainability perspective, using the STEPS matrix.

S – SOCIAL: there are essentially three issues that the swarms of polymeric microrobots, potentially adopted globally in a future scenario – not only for capturing microplastics and bacteria from polluted waters but also for sequestering other contaminants – would address. Firstly, the current wastewater treatment plants struggle to completely clean the water, leaving traces of various plastic fragments. Secondly, the problem of the biostability of drinking water is increasingly disturbed by the emergence of new pollutants in freshwater (with significant warnings coming from the United States), casting doubt on the methods currently in place to ensure its safety. Lastly, there are antibiotics and hormones, the Emerging Pollutants of our time, which are increasingly present in wastewater and which existing water treatment systems are unable to remove.

T – TECNOLOGICAL: in the future, the approach taken by the team from the University of Brno could stimulate the development of additional types of microrobot coatings capable, for instance, of attracting and capturing other waterborne pollutants simultaneously. Moreover, in the coming years, the design phase of micromachines could benefit from artificial intelligence techniques, enhancing the flight capabilities of microrobot swarms, especially in navigating specific directions within aquatic environments, particularly in settings with background disturbances.

E – ECONOMIC: envisioning a future scenario where wastewater and fresh water from rivers and lakes are managed using advanced swarms of microrobots for pollutant sequestration – whether these are materials, chemicals, or bacteria – it will be essential to establish multidisciplinary teams. These teams should include engineers specialising in microrobotics and nanotechnology, bioengineers and microbiologists, as well as environmental scientists who will lead the system’s design. They will support the local authorities responsible for protection and monitoring functions.

P – POLITICAL: in recent years, the European Union has been working on amendments and revisions to its wastewater regulations. Specifically, on 10 April 2024, new EU rules for the collection, treatment, and discharge of urban wastewater were approved. These rules tighten purification operations by adding secondary treatments before the final environmental discharge. Particular attention and rigour are given to removing certain chemicals from the water (including nitrogen, phosphorus, and PFAS) and to producer responsibility, particularly concerning human medicines and cosmetic products, covering the costs of removing their micro-pollutants from urban wastewater, as stated in the regulation. In this context, the future development of polymeric microrobot swarms for capturing microplastics and bacteria in microstructures capable of rapidly and efficiently eliminating most pollutants from wastewater would be a remarkable step forward towards achieving the EU’s goal of “zero pollution (air, water, and soil)” by 2050.

S – SUSTAINABILITY: beyond the positive impact on environmental sustainability, as mentioned, there is the significant issue of social sustainability related to access to clean water in developing countries. The United Nations has included clean water among the 17 Sustainable Development Goals (SDGs) of the 2030 Agenda (Goal 6), driven by data showing that 1.8 billion people worldwide use contaminated water sources. Microrobotics is emerging as a field of study capable of effectively intervening in water purification, where traditional technologies exhibit several weaknesses. In a possible future scenario where microrobotics is adopted globally as a central tool for water remediation, the principle of equity must be respected to ensure that all populations can benefit from it, as clean water is a universal right.

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